1. Field of the Invention
The present invention relates to a bandwidth allocation method used in a point-to-multipoint communication system in which a plurality of subsidiary units are connected to a single central unit via transmission paths and a transmission bandwidth is shared by the subsidiary units so as to send data to the central unit.
2. Description of the Related Art
Generally, in the point-to-multipoint communication system such as a LAN, a CATV network, a satellite communication network, or a subscriber access optical network, a central unit intercommunicates with a plurality of subsidiary units via a common communication path used in a coaxial communication, wireless communication, or optical fiber communication system.
The PON transmission system is an example of the point-to-multipoint communication system, in which a central unit and subsidiary units are connected via optical fibers and an optical branch section.
FIG. 2 is a block diagram showing a topology for the PON transmission system. The transmission path (i.e., optical fiber) 3 connected to the bandwidth allocating section 2 in a single central unit 1 is combined with branch optical fibers 5a, 5b, and 5c at the optical branch point 4, which are respectively connected to subsidiary units 6, 7, and 8. A transmission bandwidth (for transmitting data to the central unit 1) is allocated to each of the subsidiary units 6, 7, and 8 by the bandwidth allocating section 2.
In order to smoothly perform communication between the central unit 1 and the subsidiary units 6, 7, and 8 via the common transmission path 3, an access method for allocating the usage of the transmission path 3 to the subsidiary units 6, 7, and 8 is very important, and various kinds of the access method are known.
For example, each subsidiary unit issues a request for bandwidth allocation to the central unit within a predetermined period of time, and the central unit calculates a bandwidth (for transmitting data to the central unit) to be allocated to the subsidiary unit based on the amount of bandwidth requested by the subsidiary unit. The central unit then informs the subsidiary unit of permission for the transmission. Here, the central unit allocates bandwidths for an allocation request signal and a data signal to each subsidiary unit.
More specifically, in the first conventional method, the central unit allocates bandwidths for the allocation request signals from all subsidiary units in a specific period of time, in a manner such that the bandwidths do not overlap (or collide) with each other. According to this method, the central unit can allocate bandwidths to all subsidiary units fairly based on the allocation requests from the subsidiary units or can efficiently allocate bandwidths to all subsidiary units based on the predetermined conditions. The allocation of bandwidths for the allocation request signals is performed at regular intervals, and the remaining bandwidths are allocated for the data signals.
FIG. 3 is a diagram showing the allocation process in the first conventional method. As shown in the figure, the central unit sends a transmission permission signal (for permitting data to the central unit) to each subsidiary unit at regular intervals. According to the transmission permission signal, each subsidiary unit sends an allocation request signal to the central unit in a manner such that the sent signal reaches the central unit within the specific allocation request reception period of time.
In the transmission permission signal, the transmission start time for each signal (sent from the subsidiary unit) and the amount of the data signal (i.e., permitted amount of data) are stored, so that the allocation request signals do not overlap (or collide) with each other and the data signals of the subsidiary units also do not overlap with each other. According to such a transmission permission signal, each subsidiary unit sends the allocation request signal and the data signal.
That is, based on the allocation request signals (1, K−1), . . . , (P, K−1), received from the subsidiary units in the K−1 period (i.e., (K−1)th period), the central unit calculates allocated bandwidths in the K period (i.e., K−th period) and informs each subsidiary unit of the transmission start time for each of the allocation request signal and the data signal, and the amount of data signal.
In this process, the amounts of bandwidth to be allocated, which are requested by all subsidiary units, can be communicated to the central unit; thus, the central unit can calculate optimum bandwidths to be allocated in the next period. For example, the subsidiary unit #P (see FIG. 3) sends the allocation request signal (P, K) and the data signal (P, K) in turn in the K period, according to the transmission start times and the amount of data included in the communicated transmission permission signal (K−1).
The above-explained steps are repeated, so that the transmission bandwidth for sending data to the central unit can be dynamically allocated.
On the other hand, in the second conventional method, every time the central unit receives an allocation request from any subsidiary unit, the subsidiary unit calculates bandwidths allocated to the subsidiary unit, not in consideration of the other subsidiary units, so that bandwidths for the allocation request signal and the data signal are selected among bandwidths which have not yet been allocated. According to this method, the bandwidth allocation for the relevant subsidiary unit can be performed without awaiting allocation requests from the other subsidiary units.
FIG. 4 is a diagram showing the allocation process in the second conventional method. In this method, no specific period is defined, and the central unit 1 sends the transmission permission signals to the subsidiary units #1, . . . , #P, . . . According to the transmission permission signals, each subsidiary unit sends the allocation request signal and the data signal to the central unit 1. Every time the central unit 1 receives the allocation request signal from any subsidiary unit, the central unit 1 allocates optimum bandwidths for the allocation request signal and the data signal to be sent from the subsidiary unit, where the allocated bandwidths are selected among bandwidths which have not yet been allocated.
That is, based on the allocation request signal (P, K−1) from the subsidiary unit #P, the central unit I calculates bandwidths to be allocated next for the subsidiary unit #P and sends the transmission permission signal (P, K−1) so as to communicate the transmission start time for each of the allocation request signal and the data signal, and the amount of the data signal.
In this process, the central unit independently performs the bandwidth allocation for each subsidiary unit; thus, it is possible to quickly allocate the next bandwidth independent of the subsidiary units which have long round-trip propagation delay times, thereby reducing the delay time for starting the data transmission. For example, the subsidiary unit #P sends the allocation request signal (P, K) and the data signal (P, K) according to the transmission start time for each signal and the amount of data, which are communicated using the transmission permission signal (P, K−1). In parallel to this operation of the subsidiary unit #P, bandwidth allocation for another subsidiary unit is performed independent of the allocation for the subsidiary unit #P.
The above-explained steps are repeatedly performed, so that the transmission bandwidth for sending data to the central unit can be dynamically allocated.
In the above-explained conventional methods, it is impossible to efficiently use the transmission bandwidth while reducing the delay time for starting the transmission of the data signal (stored in the subsidiary unit) to the central unit.
In the first conventional method, the delay time can be reduced by shortening the period for bandwidth allocation. However, as shown in FIG. 5, it is necessary to receive the allocation request signals from all subsidiary units in a specific period of time; thus, the bandwidth allocation period cannot be shortened to be less than the maximum round-trip propagation delay time (refer to the length of the double-headed arrow indicated by reference numeral 100).
In the second conventional method, the bandwidth allocation period for a subsidiary unit can be shortened to the round-trip propagation delay time of the subsidiary unit, independent of the round-trip propagation delay times of the other round-trip propagation delay times. However, the bandwidth is independently allocated to each subsidiary unit without referring to the amount of bandwidths to be allocated, which are requested by other subsidiary unit; therefore, the delay time may be increased as shown in FIG. 6. In the example shown in FIG. 6, bandwidth allocation is possible from time T1 according to the round-trip propagation delay time of the subsidiary unit #1. However, owing to a section where bandwidths, which have not yet been allocated, are dispersed and thus the bandwidth allocation should be inefficiently performed (refer to the section indicated by reference numeral 200), or a section where a long bandwidth has already been allocated to a subsidiary unit and thus the delay time for starting the signal transmission of another subsidiary unit is increased (refer to the section indicated by reference numeral 300), the bandwidth cannot be efficiently used while reducing the delay time.